THE SPECIAL SENSES 233 
auditory cells, specialized nerve cells, which often bear 
delicate vibratile hairs (Fig. 145). Auditory organs of this 
general character are known among the polyps, the worms, 
the crustaceans, and the mollusks. In the common cray- 
fish the “ears” are situated in the basal segment of the 
inner antenne or feelers (Fig. 146). They consist each of 
a small sac filled with liquid in which 
are suspended several grains of sand 
or other hard bodies. The inner 
Fie. 145.—Auditory organ of a mollusk. a, audi- Fie. 146.— Antenna of 
tory nerve; b, outer wall of connective tissue ; cray - fish, with audi- 
e, cells with auditory hairs; d@, otolith.—After tory sac at buse.— 
LEyYDIa. After Hux.ey. 
surface of the sac is lined with fine auditory hairs. The 
sound waves coming through the air or water outside strike 
against this sac, which lies in a hollow on the upper or 
outer side of the antennz. The sound waves are taken up 
by the contents of the sac and stimulate the fine hairs, 
which in turn give this stimulus to the nerves which run 
from them to the principal auditory nerve and thus to the 
brain of the cray-fish. Among the insects other kinds of 
auditory organs exist. The common locust or grasshopper 
